1. Field of the Disclosure
Embodiments disclosed herein relate generally to an apparatus for controlling a back pressure control system. In another aspect, embodiments disclosed herein relate to an apparatus for controlling a plurality of back pressure control systems. In yet another aspect, embodiments disclosed herein relate to an apparatus for controlling pressure of a fluid in a wellbore.
2. Background
There are many applications in which there is a need to control the back pressure of a fluid flowing in a system. For example, in the drilling of oil wells it is customary to suspend a drill pipe in the well bore with a bit on the lower end thereof and, as the bit is rotated, to circulate a drilling fluid, such as a drilling mud, down through the interior of the drill string, out through the bit, and up the annulus of the well bore to the surface. This fluid circulation is maintained for the purpose of removing cuttings from the well bore, for cooling the bit, and for maintaining hydrostatic pressure in the well bore to control formation gases and prevent blowouts, and the like. In those cases where the weight of the drilling mud is not sufficient to contain the bottom hole pressure in the well, it becomes necessary to apply additional back pressure on the drilling mud at the surface to compensate for the lack of hydrostatic head and thereby keep the well under control. Thus, in some instances, a back pressure control device is mounted in the return flow line for the drilling fluid.
Back pressure control devices are also necessary for controlling “kicks” in the system caused by the intrusion of salt water or formation gases into the drilling fluid which may lead to a blowout condition. In these situations, sufficient additional back pressure must be imposed on the drilling fluid such that the formation fluid is contained and the well controlled until heavier fluid or mud can be circulated down the drill string and up the annulus to kill the well. It is also desirable to avoid the creation of excessive back pressures which could cause the drill string to stick, or cause damage to the formation, the well casing, or the well head equipment.
However, maintenance of an optimum back pressure on the drilling fluid is complicated by variations in certain characteristics of the drilling fluid as it passes through the back pressure control device. For example, the density of the fluid can be altered by the introduction of debris or formation gases, and/or the temperature and volume of the fluid entering the control device can change. Therefore, the desired back pressure will not be achieved until appropriate changes have been made in the throttling of the drilling fluid in response to these changed conditions. Conventional devices, such as a choke, generally require manual control of and adjustments to the back pressure control device orifice to maintain the desired back pressure. However, manual control of the throttling device involves a lag time and generally is inexact.
U.S. Pat. No. 4,355,784 discloses an apparatus and method for controlling back pressure of drilling fluid in the above environment which addresses the problems set forth above. According to this arrangement, a substantially balanced shuttle moves in a housing to control the flow and the back pressure of the drilling fluid. One end of the shuttle assembly is exposed to the pressure of the drilling fluid and its other end is exposed to the pressure of a control fluid.
U.S. Pat. No. 6,253,787 discloses a choke device that operates automatically to maintain a predetermined back pressure on the flowing fluid despite changes in fluid conditions. As described therein, to maintain accurate control of the back pressure applied during shuttling, a back pressure may be exerted on the shuttle assembly by a control fluid. The pressure of the fluid in the inlet passage acts on a corresponding end of the shuttle assembly with the same force imposed on the other end of the shuttle assembly by the control fluid.
U.S. Pat. No. 7,004,448 discloses a back pressure control system useful for operating pressures up to about 690 or 1034 bar (10,000 or 15,000 psia). The back pressure control device disclosed therein requires a wellbore pressure greater than the hydraulic set point pressure to open the valve from a fully closed position, as when in the fully closed position, the operating fluid may only act upon a portion of the operating surface area of the shuttle (reference numeral 40 in the '448 patent). However, due to control at relatively high pressures for this system, an overshoot of pressure of up to about 500 psi may be tolerated.
U.S. patent application Ser. No. 12/104,106 (U.S. Patent Application Publication No. 2009/0260698) discloses a back pressure control system useful for operating pressures up to about 103 bar (1500 psia), where a lower pressure overshoot may be required to open the valve. Such a system may be useful in Managed Pressure Drilling environments (MPDs, typically having wellbore pressures of less than 69 bar (1000 psia)).
U.S. Pat. Nos. 6,575,244 and 7,478,672 describe systems for controlling a pressure in a subterranean formation. In the '244 patent, well pressure may be controlled using an automatic choke and advanced controls (e.g., PID control). In the '672 patent, an electronic choke is controlled using a remote and a local operating panel, suitable for use in hazardous environments.
Locations where oil or gas wells are being drilled generally have limited resources available for continuous use, such as utilities including air, electricity, etc. As a result, it is not uncommon, for example, for the air supply or electrical power to a back pressure control system to be temporarily interrupted; such interruptions in back pressure system control can adversely affect drilling operations. Additional challenges exist for low-pressure (<103 bar (<1500 psia) back pressure control systems, where sustained accurate control of the well pressure is essential.
Accordingly, there exists a need for robust systems for controlling the operating pressure within a subterranean borehole